The present invention relates to active filter circuits, and more particularly to an improved pretuned single frequency bandpass filter utilizing two stages, each comprised of a differential input high performance operational amplifier.
It is well known in the micro-electronic arts, and particularly in the thick film art, that micro-electronic circuits are well adapted for construction of miniature modular devices such as audio bandpass filters. Several filter designs have been widely publicized in the past, for example, see "Operational Amplifiers" by Burr-Brown, published by McGraw-Hill. In general, in construction of such micro-electronic filters, the filter is comprised of an assemblage of resistive and capacitive elements formed on a substrate and interconnected with one or more stages each comprising a differential amplifier, such as a high performance operational amplifier. For example, integrated circuit integrated operational amplifiers similar to Fairchild Camera and Instrument Model .mu.A 741 are commonly utilized.
Various filter designs which utilize a single amplifier of the above-mentioned type have been suggested in the prior art. However, such single stage amplifiers usually manifest certain disadvantages in construction and use. For example, the filters commonly exhibit a pronounced sensitivity to the host circuit parameters. In addition, the Q of the filter comprising such an amplifier and the center frequency are usually interdependent, which is an undesirable relationship. Finally, the capacitive elements of the filter circuit may require matching for proper operation, i.e., with respect to twin T forms, for example, matching of three capacitors is required.
The disadvantages mentioned above in connection with single-stage amplifier type filters are usually avoided in more complex amplifier designs, for example in filters which employ three amplifier stages in a "state variable" or "biquad" configuration. These more complex circuits are, however, obviously more expensive to produce and hence are not as well suited for widespread use in simpler circuits.